ER-phagosome fusion defines an MHC class I cross-presentation compartment in dendritic cells

Cancer Acidity and Hypertonicity Contribute to Dysfunction of Tumor-Associated Dendritic Cells: Potential Impact on Antigen Cross-Presentation Machinery

Macrophages (MΦ) and dendritic cells (DC), major players of the mononuclear phagocyte system (MoPh), are potent antigen presenting cells that steadily sense and respond to signals from the surrounding microenvironment, leading to either immunogenic or tolerogenic outcomes. Next to classical MHC-I/MHC-II antigen-presentation pathways described in the vast majority of cell types, a subset of MoPh (CD8⁺, XCR1⁺, CLEC9A⁺, BDCA3⁺ conventional DCs in human) is endowed with a high competence to cross-present external (engulfed) antigens on MHC-I molecules to CD8⁺ T-cells. This exceptional DC function is thought to be a crucial crossroad in cytotoxic antitumor immunity and has been extensively studied in the past decades. Biophysical and biochemical fingerprints of tumor micromilieus show significant spatiotemporal differences in comparison to non-neoplastic tissue. In tumors, low pH (mainly due to extracellular lactate accumulation via the Warburg effect and via glutaminolysis) and high oncotic and osmotic pressure (resulting from tumor debris, increased extracellular matrix components but in part also triggered by nutritive aspects) are—despite fluctuations and difficulties in measurement—likely the most constant general hallmarks of tumor microenvironment. Here, we focus on the influence of acidic and hypertonic micromilieu on the capacity of DCs to cross-present tumor-specific antigens. We discuss complex and in part controversial scientific data on the interference of these factors with to date reported mechanisms of antigen uptake, processing and cross-presentation, and we highlight their potential role in cancer immune escape and poor clinical response to DC vaccines.

A guide to measuring phagosomal dynamics

Phagocytosis is an essential mechanism for immunity and homeostasis, performed by a subset of cells known as phagocytes. Upon target engulfment, de novo formation of specialized compartments termed phagosomes takes place. Phagosomes then undergo a series of fusion and fission events as they interact with the endolysosomal system and other organelles, in a dynamic process known as phagosome maturation. Because phagocytes play a key role in tissue patrolling and immune surveillance, phagosome maturation is associated with signaling pathways that link phagocytosis to antigen presentation and the development of adaptive immune responses. In addition, and depending on the nature of the cargo, phagosome integrity may be compromised, triggering additional cellular mechanisms including inflammation and autophagy. Upon completion of maturation, phagosomes enter a recently described phase: phagosome resolution, where catabolites from degraded cargo are metabolized, phagosomes are resorbed, and vesicles of phagosomal origin are recycled. Finally, phagocytes return to homeostasis and become ready for a new round of phagocytosis. Altogether, phagosome maturation and resolution encompass a series of dynamic events and organelle crosstalk that can be measured by biochemical, imaging, photoluminescence, cytometric, and immune‐based assays that will be described in this guide.

Small Molecule Enhancers of Endosome-to-Cytosol Import Augment Anti-tumor Immunity

Cross-presentation of antigens by dendritic cells (DCs) is critical for initiation of anti-tumor immune responses. Yet, key steps involved in trafficking of antigens taken up by DCs remain incompletely understood. Here, we screen 700 US Food and Drug Administration (FDA)-approved drugs and identify 37 enhancers of antigen import from endolysosomes into the cytosol. To reveal their mechanism of action, we generate proteomic organellar maps of control and drug-treated DCs (focusing on two compounds, prazosin and tamoxifen). By combining organellar mapping, quantitative proteomics, and microscopy, we conclude that import enhancers undergo lysosomal trapping leading to membrane permeation and antigen release. Enhancing antigen import facilitates cross-presentation of soluble and cell-associated antigens. Systemic administration of prazosin leads to reduced growth of MC38 tumors and to a synergistic effect with checkpoint immunotherapy in a melanoma model. Thus, inefficient antigen import into the cytosol limits antigen cross-presentation, restraining the potency of anti-tumor immune responses and efficacy of checkpoint blockers.

Interaction Between Virus-Like Particles (VLPs) and Pattern Recognition Receptors (PRRs) From Dendritic Cells (DCs): Toward Better Engineering of VLPs

Virus-like particles (VLPs) have been shown to be strong activators of dendritic cells (DCs). DCs are the most potent antigen presenting cells (APCs) and their activation prompts the priming of immunity mediators based on B and T cells. The first step for the activation of DCs is the binding of VLPs to pattern recognition receptors (PRRs) on the surface of DCs, followed by VLP internalization. Like wild-type viruses, VLPs use specific PRRs from the DC; however, these recognition interactions between VLPs and PRRs from DCs have not been thoroughly reviewed. In this review, we focused on the interaction between proteins that form VLPs and PRRs from DCs. Several proteins that form VLP contain glycosylations that allow the direct interaction with PRRs sensing carbohydrates, prompting DC maturation and leading to the development of strong adaptive immune responses. We also discussed how the knowledge of the molecular interaction between VLPs and PRRs from DCs can lead to the smart design of VLPs, whether based on the fusion of foreign epitopes or their chemical conjugation, as well as other modifications that have been shown to induce a stronger adaptive immune response and protection against infectious pathogens of importance in human and veterinary medicine. Finally, we address the use of VLPs as tools against cancer and allergic diseases.

Hemocyte phagosomal proteome is dynamically shaped by cytoskeleton remodeling and interorganellar communication with endoplasmic reticulum during phagocytosis in a marine invertebrate, Crassostrea gigas

Phagosomes are task-force organelles of innate immune systems, and evolutionary diversity and continuity abound in the protein machinery executing this coordinately regulated process. In order to clarify molecular mechanisms underlying phagocytosis, we studied phagocyte response to beads and Vibrio species, using hemocytes of the Pacific oysters (Crassostrea gigas) as a marine invertebrate model. Phagosomes from different stages of phagocytosis were isolated by density-gradient centrifugation, and more than 400 phagosome-associated proteins were subsequently identified via high-throughput quantitative proteomics. In modeling key networks of phagosomal proteins, our results support the essential roles of several processes driving phagosome formation and maturation, including cytoskeleton remodeling and signal transduction by Rab proteins. Several endoplasmic reticulum (ER)-associated proteins were identified, while live cell imaging confirms an apparent intimate interaction between the ER and phagosomes. In further quantitative proteomic analysis, the signal transducers CgRhoGDI and CgPI4K were implicated. Through experimental validation, CgRhoGDI was shown to negatively regulate actin cytoskeleton remodeling in the formation of oyster phagosomes, while CgPI4K signaling drives phagosome maturation and bacterial killing. Our current work illustrates the diversity and dynamic interplay of phagosomal proteins, providing a framework for better understanding host-microbe interactions during phagosome activities in under-examined invertebrate species.

MHC class I and II peptide homology regulates the cellular immune response

Mammalian immune responses are initiated by "danger" signals--immutable molecular structures known as PAMPs. When detected by fixed, germline encoded receptors, pathogen-associated molecular pattern (PAMPs) subsequently inform the polarization of downstream adaptive responses depending upon identity and localization of the PAMP. Here, we report the existence of a completely novel "PAMP" that is not a molecular structure but an antigenic pattern. This pattern--the incidence of peptide epitopes with stretches of 100% sequence identity bound to both dendritic cell (DC) major histocompatibility (MHC) class I and MHC class II--strongly induces T_H 1 immune polarization and activation of the cellular immune response. Inherent in the existence of this PAMP is the concomitant existence of a molecular sensor complex with the ability to scan and compare amino acid sequence identities of bound class I and II peptides. We provide substantial evidence implicating the multienzyme aminoacyl-tRNA synthetase (mARS) complex and its AIMp1 structural component as the key constituents of this complex. The results demonstrate a wholly novel mechanism by which T-helper (T_H ) polarization is governed and provide critical information for the design of vaccination strategies intended to provoke cell-mediated immunity.

Endoplasmic Reticulum-Associated Degradation-Dependent Processing in Cross-Presentation and Its Potential for Dendritic Cell Vaccinations: A Review

While the success of dendritic cell (DC) vaccination largely depends on cross-presentation (CP) efficiency, the precise molecular mechanism of CP is not yet characterized. Recent research revealed that endoplasmic reticulum (ER)-associated degradation (ERAD), which was first identified as part of the protein quality control system in the ER, plays a pivotal role in the processing of extracellular proteins in CP. The discovery of ERAD-dependent processing strongly suggests that the properties of extracellular antigens are one of the keys to effective DC vaccination, in addition to DC subsets and the maturation of these cells. In this review, we address recent advances in CP, focusing on the molecular mechanisms of the ERAD-dependent processing of extracellular proteins. As ERAD itself and the ERAD-dependent processing in CP share cellular machinery, enhancing the recognition of extracellular proteins, such as the ERAD substrate, by ex vivo methods may serve to improve the efficacy of DC vaccination.

The GTPase Rab39a promotes phagosome maturation into MHC-I antigen-presenting compartments

For CD8 T lymphocytes to mount responses to cancer and virally-infected cells, dendritic cells must capture antigens present in tissues and display them as peptides bound to MHC-I molecules. This is most often accomplished through a pathway called antigen cross-presentation (XPT). Here, we report that the vesicular trafficking protein Rab39a is needed for optimal cross-presentation by dendritic cells in vitro and cross-priming of CD8 T cells in vivo. Without Rab39a, MHC-I presentation of intraphagosomal peptides is inhibited, indicating that Rab39a converts phagosomes into peptide-loading compartments. In this process, Rab39a promotes the delivery of MHC-I molecules from the endoplasmic reticulum (ER) to phagosomes, and increases the levels of peptide-empty MHC-I conformers that can be loaded with peptide in this compartment. Rab39a also increases the levels of Sec22b and NOX2, previously recognized to participate in cross-presentation, on phagosomes, thereby filling in a missing link into how phagosomes mature into cross-presenting vesicles.

Splenic Red Pulp Macrophages Cross-Prime Early Effector CTL That Provide Rapid Defense against Viral Infections

Cross-presentation allows dendritic cells (DCs) to present peptides derived from endocytosed Ags on MHC class I molecules, which is important for activating CTL against viral infections and tumors. Type 1 classical DCs (cDC1), which depend on the transcription factor Batf3, are considered the main cross-presenting cells. In this study, we report that soluble Ags are efficiently cross-presented also by transcription factor SpiC-dependent red pulp macrophages (RPM) of the spleen. In contrast to cDC1, RPM used the mannose receptor for Ag uptake and employed the proteasome- and TAP-dependent cytosolic cross-presentation pathway, previously shown to be used in vitro by bone marrow-derived DCs. In an in vivo vaccination model, both cDC1 and RPM cross-primed CTL efficiently but with distinct kinetics. Within a few days, RPM induced very early effector CTL of a distinct phenotype (Ly6A/E⁺ Ly6C⁽⁺⁾ KLRG1^- CD127^- CX₃CR1^- Grz-B⁺). In an adenoviral infection model, such CTL contained the early viral spread, whereas cDC1 induced short-lived effector CTL that eventually cleared the virus. RPM-induced early effector CTL also contributed to the endogenous antiviral response but not to CTL memory generation. In conclusion, RPM can contribute to antiviral immunity by generating a rapid CTL defense force that contains the virus until cDC1-induced CTL are available to eliminate it. This function can be harnessed for improving vaccination strategies aimed at inducing CTL.

Nanoscale Coordination Polymer Based Nanovaccine for Tumor Immunotherapy

Tumor vaccines to induce robust immunity for cancer treatment have attracted tremendous interests in cancer immunotherapy. In this work, a type of cancer vaccine is prepared by using nanoscale coordination polymer (NCP) formed between Mn²⁺ ions and a nucleotide oligomerization binding domain 1 (Nod1) agonist, meso-2,6-diaminopimelic acid (DAP), as the organic ligand, to encapsulate a model protein antigen, ovalbumin (OVA). The obtained OVA@Mn-DAP nanoparticles could act as an effective tumor vaccine to promote the maturation of dendritic cells (DCs) as well as their antigen cross-presentation via increasing the cellular uptake of antigen and stimulating Nod1 pathway with DAP. Such OVA@Mn-DAP vaccine could migrate into lymph nodes after local injection, as revealed by in vivo magnetic resonance (MR) and fluorescence imaging. Importantly, vaccination with OVA@Mn-DAP could not only offer prophylactic to protect mice from challenged B16-OVA tumors but also result in significant therapeutic effect to inhibit growth of already-established tumors if in combination with anti-programmed cell death protein 1 antibody (α-PD-1) immune checkpoint blockade therapy. Therefore, this work presents an innovative platform to construct effective nanovaccine for tumor immunotherapy.

The role of MHC class I recycling and Arf6 in cross-presentation by murine dendritic cells

Cross-presentation by MHC class I molecules (MHC-I) is critical for priming of cytotoxic T cells. Peptides derived from cross-presented antigens can be loaded on MHC-I in the endoplasmic reticulum and in endocytic or phagocytic compartments of murine DCs. However, the origin of MHC-I in the latter compartments is poorly understood. Recently, Rab22-dependent MHC-I recycling through a Rab11+ compartment has been suggested to be implicated in cross-presentation. We have examined the existence of MHC-I recycling and the role of Arf6, described to regulate recycling in nonprofessional antigen presenting cells, in murine DCs. We confirm folded MHC-I accumulation in a juxtanuclear Rab11+ compartment and partially localize Arf6 to this compartment. MHC-I undergo fast recycling, however, both folded and unfolded internalized MHC-I fail to recycle to the Rab11+Arf6+ compartment. Therefore, the source of MHC-I molecules in DC endocytic compartments remains to be identified. Functionally, depletion of Arf6 compromises cross-presentation of immune complexes but not of soluble, phagocytosed or mannose receptor-targeted antigen, suggesting a role of Fc receptor-regulated Arf6 trafficking in cross-presentation of immune complexes.

Distinct Subcellular Compartments of Dendritic Cells Used for Cross-Presentation

Dendritic cells (DCs) present exogenous protein-derived peptides on major histocompatibility complex class I molecules to prime naïve CD8⁺ T cells. This DC specific ability, called cross-presentation (CP), is important for the activation of cell-mediated immunity and the induction of self-tolerance. Recent research revealed that endoplasmic reticulum-associated degradation (ERAD), which was first identified as a part of the unfolded protein response—a quality control system in the ER—plays a pivotal role in the processing of exogenous proteins in CP. Moreover, DCs express a variety of immuno-modulatory molecules and cytokines to regulate T cell activation in response to the environment. Although both CP and immuno-modulation are indispensable, contrasting ER conditions are required for their correct activity. Since ERAD substrates are unfolded proteins, their accumulation may result in ER stress, impaired cell homeostasis, and eventually apoptosis. In contrast, activation of the unfolded protein response should be inhibited for DCs to express immuno-modulatory molecules and cytokines. Here, we review recent advances on antigen CP, focusing on intracellular transport routes for exogenous antigens and distinctive subcellular compartments involved in ERAD.

Cell and fluid sampling microneedle patches for monitoring skin-resident immunity

Important cell populations reside within tissues and are not accessed by traditional blood draws used to monitor the immune system. To address this issue at an essential barrier tissue, the skin, we created a microneedle-based technology for longitudinal sampling of cells and interstitial fluid, enabling minimally invasive parallel monitoring of immune responses. Solid microneedle projections were coated by a cross-linked biocompatible polymer, which swells upon skin insertion, forming a porous matrix for local leukocyte infiltration. By embedding molecular adjuvants and specific antigens encapsulated in nanocapsules within the hydrogel coating, antigen-specific lymphocytes can be enriched in the recovered cell population, allowing for subsequent detailed phenotypic and functional analysis. We demonstrate this approach in mice immunized with a model protein antigen or infected in the skin with vaccinia virus. After vaccination or infection, sampling microneedles allowed tissue-resident memory T cells (T_RMs) to be longitudinally monitored in the skin for many months, during which time the antigen-specific T cell population in systemic circulation contracted to low or undetectable counts. Sampling microneedles did not change the immune status of naïve or antigen-exposed animals. We also validated the ability of cell sampling using human skin samples. This approach may be useful in vaccines and immunotherapies to temporally query T_RM populations or as a diagnostic platform to sample for biomarkers in chronic inflammatory and autoimmune disorders, allowing information previously accessible only via invasive biopsies to be obtained in a minimally invasive manner from the skin or other mucosal tissues.

Immunomodulatory Nanosystems

Immunotherapy has emerged as an effective strategy for the prevention and treatment of a variety of diseases, including cancer, infectious diseases, inflammatory diseases, and autoimmune diseases. Immunomodulatory nanosystems can readily improve the therapeutic effects and simultaneously overcome many obstacles facing the treatment method, such as inadequate immune stimulation, off-target side effects, and bioactivity loss of immune agents during circulation. In recent years, researchers have continuously developed nanomaterials with new structures, properties, and functions. This Review provides the most recent advances of nanotechnology for immunostimulation and immunosuppression. In cancer immunotherapy, nanosystems play an essential role in immune cell activation and tumor microenvironment modulation, as well as combination with other antitumor approaches. In infectious diseases, many encouraging outcomes from using nanomaterial vaccines against viral and bacterial infections have been reported. In addition, nanoparticles also potentiate the effects of immunosuppressive immune cells for the treatment of inflammatory and autoimmune diseases. Finally, the challenges and prospects of applying nanotechnology to modulate immunotherapy are discussed.

Antigen processing and presentation

Dendritic cells are at the center of immune responses. They are defined by their ability to sense the environment, take up and process antigen, migrate to secondary lymphoid organs, where they present antigens to the adaptive immune system. In particular, they present lipids and proteins from pathogens, which they encountered in peripheral tissues, to T cells in order to induce a specific effector immune response. These complex antigens need to be broken down into peptides of a certain length in association with Major Histocompatibility Complex (MHC) molecules. Presentation of MHC/antigen complexes alongside costimulatory molecules and secretion of proinflammatory cytokines will induce an appropriate immune response. This interaction between dendritic cells and T cells takes place at defined locations within secondary lymphoid organs. In this review, we discuss the current knowledge and recent advances on the cellular and molecular mechanisms that underlie antigen processing and the subsequent presentation to T lymphocytes.

Antigen cross-presentation: proteasome location, location, location

Our understanding of the mechanisms by which peptides from proteins present in phagosomes and endosomes are processed and presented on MHC class I molecules, in a pathway called cross-presentation, is still incomplete. One of the main questions arising from currently proposed models is how do proteins in the phagosome lumen reach the proteasome in the cytoplasm to be processed properly. In this issue of The EMBO Journal, Sengupta et al (2019) present evidence for a surprising turn of events where, in fact, the proteasome acts within the lumen of endosomes and phagosomes.

Proteasomal degradation within endocytic organelles mediates antigen cross-presentation

During MHC-I-restricted antigen processing, peptides generated by cytosolic proteasomes are translocated by the transporter associated with antigen processing (TAP) into the endoplasmic reticulum, where they bind to newly synthesized MHC-I molecules. Dendritic cells and other cell types can also generate MHC-I complexes with peptides derived from internalized proteins, a process called cross-presentation. Here, we show that active proteasomes within cross-presenting cell phagosomes can generate these peptides. Active proteasomes are detectable within endocytic compartments in mouse bone marrow-derived dendritic cells. In TAP-deficient mouse dendritic cells, cross-presentation is enhanced by the introduction of human β₂ -microglobulin, which increases surface expression of MHC-I and suggests a role for recycling MHC-I molecules. In addition, surface MHC-I can be reduced by proteasome inhibition and stabilized by MHC-I-restricted peptides. This is consistent with constitutive proteasome-dependent but TAP-independent peptide loading in the endocytic pathway. Rab-GTPase mutants that restrain phagosome maturation increase proteasome recruitment and enhance TAP-independent cross-presentation. Thus, phagosomal/endosomal binding of peptides locally generated by proteasomes allows cross-presentation to generate MHC-I-peptide complexes identical to those produced by conventional antigen processing.

Neutrophils in innate immunity and systems biology-level approaches

The innate immune system is the first line of host defense against invading microorganisms. Polymorphonuclear leukocytes (PMNs or neutrophils) are the most abundant leukocyte in humans and essential to the innate immune response against invading pathogens. Compared to the acquired immune response, which requires time to develop and is dependent on previous interaction with specific microbes, the ability of neutrophils to kill microorganisms is immediate, nonspecific, and not dependent on previous exposure to microorganisms. Historically, studies of PMN-pathogen interaction focused on the events leading to killing of microorganisms, such as recruitment/chemotaxis, transmigration, phagocytosis, and activation, whereas postphagocytosis sequelae were infrequently considered. In addition, it was widely accepted that human neutrophils possessed limited capacity for new gene transcription and thus, relatively little biosynthetic capacity. This notion has changed dramatically within the past 20 years. Further, there is now more effort directed to understand the events occurring in PMNs after killing of microbes. Herein, we give an updated review of the systems biology-level approaches that have been used to gain an enhanced view of the role of neutrophils during host-pathogen interaction and neutrophil-mediated diseases. We anticipate that these and future systems-level studies will continue to provide information important for understanding, treatment, and control of diseases caused by pathogenic microorganisms. This article is categorized under: Physiology > Organismal Responses to Environment Physiology > Mammalian Physiology in Health and Disease Biological Mechanisms > Cell Fates.

Nanovaccine based on a protein-delivering dendrimer for effective antigen cross-presentation and cancer immunotherapy

Cancer vaccines for prevention and treatment of tumors have attracted tremendous interests as a type of cancer immunotherapy strategy. A major challenge in achieving robust T-cell responses to destruct tumor cells after vaccination is the abilities of antigen cross-presentation for antigen-presenting cells (APCs) such as dendritic cells (DCs). Herein, we demonstrate that a polyamidoamine dendrimer modified with guanidinobenzoic acid (DGBA) could serve as an effective protein carrier to enable delivery of protein antigen, thereby leading to effective antigen cross-presentation by DCs. With ovalbumin (OVA) as the model antigen and unmethylated cytosine-guanine dinucleotides (CpG) as the adjuvant, a unique type of tumor vaccine is formulated. Importantly, such DGBA-OVA-CpG nanovaccine can induce robust antigen-specific cellular immunities and further demonstrates outstanding prophylactic efficacy against B16-OVA melanoma. More significantly, the nanovaccine shows excellent therapeutic effect to treat established B16-OVA melanoma when used in combination with the programmed cell death protein 1 (PD-1) checkpoint-blockade immunotherapy. This study presents the great promises of employing rationally engineered cytosolic protein carriers for the development of tumor vaccines to achieve effective cancer immunotherapy.

DNA nanodevices map enzymatic activity in organelles

Cellular reporters of enzyme activity are based on either fluorescent proteins or small molecules. Such reporters provide information corresponding to wherever inside cells the enzyme is maximally active and preclude minor populations present in subcellular compartments. Here we describe a chemical imaging strategy to selectively interrogate minor, subcellular pools of enzymatic activity. This new technology confines the detection chemistry to a designated organelle, enabling imaging of enzymatic cleavage exclusively within the organelle. We have thus quantitatively mapped disulfide reduction exclusively in endosomes in Caenorhabditis elegans and identified that exchange is mediated by minor populations of the enzymes PDI-3 and TRX-1 resident in endosomes. Impeding intra-endosomal disulfide reduction by knocking down TRX-1 protects nematodes from infection by Corynebacterium diphtheriae, revealing the importance of this minor pool of endosomal TRX-1. TRX-1 also mediates endosomal disulfide reduction in human cells. A range of enzymatic cleavage reactions in organelles are amenable to analysis by this new reporter strategy.

A personal retrospective on the mechanisms of antigen processing

My intention here is to describe the history of the molecular aspects of the antigen processing field from a personal perspective, beginning with the early identification of the species that we now know as MHC class I and MHC class II molecules, to the recognition that their stable surface expression and detection by T cells depends on peptide association, and to the unraveling of the biochemical and cell biological mechanisms that regulate peptide binding. One goal is to highlight the role that serendipity or, more colloquially, pure blind luck can play in advancing the research enterprise when it is combined with an appropriately receptive mind. This is not intended to be an overarching review, and because of my own work I focus primarily on studies of the human MHC. This means that I neglect the work of many other individuals who made advances in other species, particularly those who produced the many knockout mouse strains used to demonstrate the importance of the antigen processing machinery for initiating immune responses. I apologize in advance to colleagues around the globe whose contributions I deal with inadequately for these reasons, and to those whose foundational work is now firmly established in text books and therefore not cited. So many individuals have worked to advance the field that giving all of them the credit they deserve is almost impossible. I have attempted, while focusing on work from my own laboratory, to point out contemporaneous or sometimes earlier advances made by others. Much of the success of my own laboratory came because we simultaneously worked on both the MHC class I and class II systems and used the findings in one area to inform the other, but mainly it depended on the extraordinary group of students and fellows who have worked on these projects over the years. To those who worked in other areas who are not mentioned here, rest assured that I appreciate your efforts just as much.

Regulation of Antigen Export to the Cytosol During Cross-Presentation

Cross-priming refers to the induction of primary cytotoxic CD8⁺ T cell responses to antigens that are not expressed in antigen presenting cells (APCs) responsible for T cell priming. Cross-priming is achieved through cross-presentation of exogenous antigens derived from tumors, extracellular pathogens or infected neighboring cells on Major Histocompatibility Complex (MHC) class I molecules. Despite extensive research efforts to understand the intracellular pathways involved in antigen cross-presentation, certain critical steps remain elusive and controversial. Here we review recent advances on antigen cross-presentation, focusing on the mechanisms involved in antigen export to the cytosol, a crucial step of this pathway.

Endosomal and Phagosomal SNAREs

The soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) protein family is of vital importance for organelle communication. The complexing of cognate SNARE members present in both the donor and target organellar membranes drives the membrane fusion required for intracellular transport. In the endocytic route, SNARE proteins mediate trafficking between endosomes and phagosomes with other endosomes, lysosomes, the Golgi apparatus, the plasma membrane, and the endoplasmic reticulum. The goal of this review is to provide an overview of the SNAREs involved in endosomal and phagosomal trafficking. Of the 38 SNAREs present in humans, 30 have been identified at endosomes and/or phagosomes. Many of these SNAREs are targeted by viruses and intracellular pathogens, which thereby reroute intracellular transport for gaining access to nutrients, preventing their degradation, and avoiding their detection by the immune system. A fascinating picture is emerging of a complex transport network with multiple SNAREs being involved in consecutive trafficking routes.

Regulation of phagosome functions by post-translational modifications: a new paradigm

Phagosomes are highly dynamic organelles formed by the uptake of particles through phagocytic innate immune cells such as macrophages. Their key roles in microbe elimination and antigen presentation make them essential for innate and adaptive immunity. However, phagosomes are also important for tissue homeostasis as even in healthy individuals billions of dead cells are phagocytosed each day. In this short review, we highlight how the use of latex beads as inert baits for phagocytosis and subsequent analysis by proteomics has changed our understanding of the phagosome. We further discuss recent data on post-translational modifications such as phosphorylation and ubiquitylation that regulate phagosome functions and demonstrate that the phagosome is not only a 'degradative organelle' but also serves as a subcellular signalling platform.

The Role of Type 1 Conventional Dendritic Cells in Cancer Immunity

Dendritic cells (DCs) are key orchestrators of immune responses. A specific DC subset, conventional type 1 DCs (cDC1s), has been recently associated with human cancer patient survival and, in preclinical models, is critical for the spontaneous rejection of immunogenic cancers and for the success of T cell-based immunotherapies. The unique role of cDC1 reflects the ability to initiate de novo T cell responses after migrating to tumor-draining lymph nodes, as well as to attract T cells, secrete cytokines, and present tumor antigens within the tumor microenvironment, enhancing local cytotoxic T cell function. Strategies aimed at increasing cDC1 abundance in tumors and enhancing their functionality provide attractive new avenues to boost anti-tumor immunity and overcome resistance to cancer immunotherapies.

Bone marrow endothelial cells sustain a tumor-specific CD8+ T cell subset with suppressive function in myeloma patients

Endothelial cells (EC) line the bone marrow microvasculature and are in close contact with CD8⁺ T cells that come and go across the permeable capillaries. Because of these intimate interactions, we investigated the capacity of EC to act as antigen-presenting cells (APC) and modulate CD8⁺ T cell activation and proliferation in bone marrow of patients with multiple myeloma (MM) and monoclonal gammopathy of undetermined significance. We found that EC from MM patients show a phenotype of semi-professional APC given that they express low levels of the co-stimulatory molecules CD40, CD80 and CD86, and of the inducible co-stimulator ligand (ICOSL). In addition, they do not undergo the strong switch from immunoproteasome to standard proteasome subunit expression which is typical of mature professional APC such as dendritic cells. EC can trap and present antigen to CD8⁺ T cells, stimulating a central memory CD8⁺ T cell population that expresses Foxp3 and produces high amounts of IL-10 and TGF-β. Another CD8⁺ T cell population is stimulated by professional APC, produces IFN-γ, and exerts antitumor activity. Thus, two distinct CD8⁺ T cell populations coexist in the bone marrow of MM patients: the first population is sustained by EC, expresses Foxp3, produces IL-10 and TGF-β, and exerts pro-tumor activity by negatively regulating the second population. This study adds new insight into the role that EC play in MM biology and describes an additional immune regulatory mechanism that inhibits the development of antitumor immunity and may impair the success of cancer immunotherapy.

Overcoming Tumor-Induced Immune Suppression: From Relieving Inhibition to Providing Costimulation with T Cell Agonists

Recent advancements in T-cell biology and antibody engineering have opened doors to significant improvements in cancer immunotherapy. Initial success with monoclonal antibodies targeting key receptors that inhibit T-cell function such as cytotoxic T lymphocyte antigen 4 (CTLA-4) and programmed death-ligand 1 (PD-1) have demonstrated the potency of this new class of therapy, highlighted by long-term complete responses for metastatic cancers once thought incurable. However, only a subset of patients responds to checkpoint blockade because of a multitude of factors, including an immunosuppressive tumor microenvironment and the mutational burden of the cancer. Novel antibodies, as well as ligand-immunoglobulin fusion proteins that target costimulatory immune receptors, are being developed and tested in clinical trials to further enhance the anti-tumor immune response. Many of these costimulatory receptors are in the tumor necrosis factor receptor superfamily (TNFRSF) and are expressed on multiple immune cell types, including inhibitory cells. While TNFRSFs signal through common pathways, the outcome of targeting different receptors depends on the functional status of the cell types expressing the relevant receptors. In this review, we discuss the current state of targeted costimulatory immunotherapy.

The role of HLA-B*27 in spondyloarthritis

The mechanism by which HLA-B*27 predisposes to spondyloarthritis remains unresolved. Arthritogenic peptides have not been defined in humans and are not involved in experimental models of spondyloarthritis. Aberrant properties of HLA-B*27 can activate the IL-23/IL-17 axis in HLA-B*27 transgenic rats and humans. In HLA-B*27-independent rodent models, spondyloarthritis can be driven by IL-23 triggering entheseal-resident CD4^-/CD8^- T cells or CD4⁺ Th17 T cells. These findings point toward noncanonical mechanisms linking HLA-B*27 to the disease and provide a potential explanation for HLA-B*27-negative spondyloarthritis. Gut microbial dysbiosis may be important in the development of spondyloarthritis. HLA-B*27-induced changes in gut microbiota are complex and suggest an ecological model of dysbiosis in rodents. The importance of the IL-23/IL-17 axis in ankylosing spondylitis has been demonstrated by studies showing efficacy of IL-17. Although deciphering the precise role(s) of HLA-B*27 in disease requires further investigation, considerable progress has been made in understanding this complex relationship.

A novel probe to assess cytosolic entry of exogenous proteins

Dendritic cells use a specialized pathway called cross-presentation to activate CD8+ T cells by presenting peptides from exogenous protein antigens on major histocompatibility complex class I molecules. Considerable evidence suggests that internalized antigens cross endocytic membranes to access cytosolic proteasomes for processing. The mechanism of protein dislocation represents a major unsolved problem. Here we describe the development of a sensitive reporter substrate, an N-glycosylated variant of Renilla luciferase fused to the Fc region of human IgG1. The luciferase variant is designed to be enzymatically inactive when glycosylated, but active after the asparagine to aspartic acid conversion that occurs upon deglycosylation by the cytosolic enzyme N-glycanase-1. The generation of cytosolic luminescence depends on internalization, deglycosylation, the cytosolic AAA-ATPase VCP/p97, and the cytosolic chaperone HSP90. By incorporating a T cell epitope into the fusion protein, we demonstrate that antigen dislocation into the cytosol is the rate limiting step in cross-presentation.

Current Concepts of Antigen Cross-Presentation

Dendritic cells have the ability to efficiently present internalized antigens on major histocompatibility complex (MHC) I molecules. This process is termed cross-presentation and is important role in the generation of an immune response against viruses and tumors, after vaccinations or in the induction of immune tolerance. The molecular mechanisms enabling cross-presentation have been topic of intense debate since many years. However, a clear view on these mechanisms remains difficult, partially due to important remaining questions, controversial results and discussions. Here, we give an overview of the current concepts of antigen cross-presentation and focus on a description of the major cross-presentation pathways, the role of retarded antigen degradation for efficient cross-presentation, the dislocation of antigens from endosomal compartment into the cytosol, the reverse transport of proteasome-derived peptides for loading on MHC I and the translocation of the cross-presentation machinery from the ER to endosomes. We try to highlight recent advances, discuss some of the controversial data and point out some of the major open questions in the field.

HMGB1 released from intestinal epithelia damaged by cholera toxin adjuvant contributes to activation of mucosal dendritic cells and induction of intestinal cytotoxic T lymphocytes and IgA

Cholera toxin (CT) is a potent mucosal adjuvant and oral administration of ovalbumin (OVA) antigens plus CT induces OVA-specific CD8+ cytotoxic T lymphocytes (CTLs) and IgA production in intestinal mucosa. However, the mechanisms of induction of these immune responses remain unknown. Intestinal OVA-specific CD8+ CTLs were not induced by oral administration of the CT active (CTA) or CT binding (CTB) subunit as an adjuvant and CD11c+ DCs were involved in cross-priming of intestinal CTLs. CD8+CD103+CD11c+CD11b-DCs and DCIR2+CD103+CD11c+CD11b+ DCs were distributed in the intestinal lamina propria and mesenteric lymph nodes, both DC subsets expressed DEC-205, and the expression of co-stimulatory molecules such as CD80 and CD86 was enhanced in both DC subsets after oral administration of intact CT but not the CTA or CTB subunit. Intestinal DCs activated by the oral administration of OVA plus CT cross-presented OVA antigens and DCs that captured OVA antigen through DEC-205, but not DCIR2, could cross-present antigen. We found that oral administration of intact CT, but not the CTA or CTB subunit, enhanced cell death, cytoplasmic expression of high-mobility group box 1 protein (HMGB1) in epithelial cell adhesion molecule (EpCAM)+CD45- intestinal epithelial cells (IECs), and HMGB1 levels in fecal extracts. HMGB1 dose-dependently enhanced the expression of CD80 and CD86 on DCs in vitro, and intravenous or oral administration of glycyrrhizin, an HMGB1 inhibitor, significantly suppressed activation of mucosal DCs and induction of intestinal OVA-specific CTLs and IgA by oral CT administration. These results showed that oral administration of intact CT triggers epithelial cell death in the gut and the release of HMGB1 from damaged IECs, and that the released HMGB1 may mediate activation of mucosal DCs and induction of CTLs and IgA in the intestine.

Antigen presentation unfolded: identifying convergence points between the UPR and antigen presentation pathways

The unfolded protein response (UPR) is an adaptive response meant to restore endoplasmic reticulum homeostasis in conditions of ER stress that subvert the folding capacity of the cell. Over the past few years, it has become clear that the functions of the UPR stretch far beyond their canonical role and intersect with seemingly unrelated functions such as innate immunity and antigen presentation. The aim of the present review is to dissect how the UPR interferes directly and indirectly with the major processes of MHC-I and MHC-II antigen presentation.

Role of calcium permeable channels in dendritic cell migration

Calcium ion (Ca²⁺) is an essential second messenger involved in multiple cellular and subcellular processes. Ca²⁺ can be released and sensed globally or locally within cells, providing complex signals of variable amplitudes and time-scales. The key function of Ca²⁺ in the regulation of acto-myosin contractility has provided a simple explanation for its role in the regulation of immune cell migration. However, many questions remain, including the identity of the Ca²⁺ stores, channels and upstream signals involved in this process. Here, we focus on dendritic cells (DCs), because their immune sentinel function heavily relies on their capacity to migrate within tissues and later on between tissues and lymphoid organs. Deciphering the mechanisms by which cytoplasmic Ca²⁺ regulate DC migration should shed light on their role in initiating and tuning immune responses.

Perpetual complexity: predicting human CD8+ T-cell responses to pathogenic peptides

The accurate prediction of human CD8⁺ T-cell epitopes has great potential clinical and translational implications in the context of infection, cancer and autoimmunity. Prediction algorithms have traditionally focused on calculated peptide affinity for the binding groove of MHC-I. However, over the years it has become increasingly clear that the ultimate T-cell recognition of MHC-I-bound peptides is governed by many contributing factors within the complex antigen presentation pathway. Recent advances in next-generation sequencing and immunnopeptidomics have increased the precision of HLA-I sub-allele classification, and have led to the discovery of peptide processing events and individual allele-specific binding preferences. Here, we review some of the discoveries that initiated the development of peptide prediction algorithms, and outline some of the current available online tools for CD8⁺ T-cell epitope prediction.

Regulation of the Cell Biology of Antigen Cross-Presentation

Antigen cross-presentation is an adaptation of the cellular process of loading MHC-I molecules with endogenous peptides during their biosynthesis within the endoplasmic reticulum. Cross-presented peptides derive from internalized proteins, microbial pathogens, and transformed or dying cells. The physical separation of internalized cargo from the endoplasmic reticulum, where the machinery for assembling peptide-MHC-I complexes resides, poses a challenge. To solve this problem, deliberate rewiring of organelle communication within cells is necessary to prepare for cross-presentation, and different endocytic receptors and vesicular traffic patterns customize the emergent cross-presentation compartment to the nature of the peptide source. Three distinct pathways of vesicular traffic converge to form the ideal cross-presentation compartment, each regulated differently to supply a unique component that enables cross-presentation of a diverse repertoire of peptides. Delivery of centerpiece MHC-I molecules is the critical step regulated by microbe-sensitive Toll-like receptors. Defining the subcellular sources of MHC-I and identifying sites of peptide loading during cross-presentation remain key challenges.

Endocytosed soluble cowpox virus protein CPXV012 inhibits antigen cross-presentation in human monocyte-derived dendritic cells

Viruses may interfere with the MHC class I antigen presentation pathway in order to avoid CD8⁺ T cell-mediated immunity. A key target within this pathway is the peptide transporter TAP. This transporter plays a central role in MHC class I-mediated peptide presentation of endogenous antigens. In addition, TAP plays a role in antigen cross-presentation of exogenously derived antigens by dendritic cells (DCs). In this study, a soluble form of the cowpox virus TAP inhibitor CPXV012 is synthesized for exogenous delivery into the antigen cross-presentation route of human monocyte-derived (mo)DCs. We show that soluble CPXV012 localizes to TAP⁺ compartments that carry internalized antigen and is a potent inhibitor of antigen cross-presentation. CPXV012 stimulates the prolonged deposition of antigen fragments in storage compartments of moDCs, as a result of reduced endosomal acidification and reduced antigen proteolysis when soluble CPXV012 is present. Thus, a dual function can be proposed for CPXV012: inhibition of TAP-mediated peptide transport and inhibition of endosomal antigen degradation. We propose this second function for soluble CPXV012 can serve to interfere with antigen cross-presentation in a peptide transport-independent manner.

Differential requirement of Rab22a for the recruitment of ER-derived proteins to phagosomes and endosomes in dendritic cells

The recruitment of endoplasmic reticulum (ER) components to dendritic cell (DC) phagosomes and endosomes is a crucial event to achieve efficient cross-presentation of exogenous antigens. We have previously identified the small GTPase Rab22a as a key regulator of MHC-I trafficking and antigen cross-presentation by DCs. In this study we show that low expression of Rab22a does not prevent the normal delivery of ER-derived proteins to DC phagosomes. In contrast, the presence of these proteins was diminished in endosomes labelled with a fluid phase marker. These observations were confirmed by a functional assay that assesses the translocation of a soluble protein to the cytosol. Interestingly, we also demonstrate that early endosomal maturation is altered in Rab22a deficient DCs. Our results indicate that Rab22a plays a major role in endosomal function and highlight the importance of studying the endocytic and phagocytic pathways separately in DCs.

Assessing the Phagosome Proteome by Quantitative Mass Spectrometry

Phagocytosis is the process that engulfs particles in vesicles called phagosomes that are trafficked through a series of maturation steps, culminating in the destruction of the internalized cargo. Because phagosomes are in direct contact with the particle and undergo constant fusion and fission events with other organelles, characterization of the phagosomal proteome is a powerful tool to understand mechanisms controlling innate immunity as well as vesicle trafficking. The ability to isolate highly pure phagosomes through the use of latex beads led to an extensive use of proteomics to study phagosomes under different stimuli. Thousands of different proteins have been identified and quantified, revealing new properties and shedding new light on the dynamics and composition of maturing phagosomes and innate immunity mechanisms. In this chapter, we describe how quantitative-based proteomic methods such as label-free, dimethyl labeling or Tandem Mass Tag (TMT) labeling can be applied for the characterization of protein composition and translocation during maturation of phagosomes in macrophages.

STING, DCs and the link between innate and adaptive tumor immunity

Cancer and the immune system are intimately related. Much of the bulk of tumors is comprised of stromal leukocytes with immune functions, which serve to both promote and inhibit tumor growth, invasion and metastasis. The T lymphocytes of the adaptive immune system are essential for tumor immunity, and these T cells are generated by cross-priming against tumor associated antigens. Dendritic cells (DCs) are essential in this process, serving as the cellular link between innate and adaptive immunity. As a prerequisite for priming of adaptive immune responses, DCs must take up tumor antigens, process them and present them in the context of the major histocompatibility complex (MHC). DCs also serve as sensors of innate activation signals from cancer that are necessary for their activation and effective priming of cancer specific T cells. Here we discuss the role of DCs in the sensing of cancer and in priming the adaptive response against tumors. Furthermore, we present the essential role of the Stimulator of Interferon Genes (STING) signaling pathway in producing type I interferons (IFNs) that are essential in this process.

STIM1 promotes migration, phagosomal maturation and antigen cross-presentation in dendritic cells

Antigen cross-presentation by dendritic cells (DC) stimulates cytotoxic T cell activation to promote immunity to intracellular pathogens, viruses and cancer. Phagocytosed antigens generate potent T cell responses, but the signalling and trafficking pathways regulating their cross-presentation are unclear. Here, we show that ablation of the store-operated-Ca²⁺-entry regulator STIM1 in mouse myeloid cells impairs cross-presentation and DC migration in vivo and in vitro. Stim1 ablation reduces Ca²⁺ signals, cross-presentation, and chemotaxis in mouse bone-marrow-derived DCs without altering cell differentiation, maturation or phagocytic capacity. Phagosomal pH homoeostasis and ROS production are unaffected by STIM1 deficiency, but phagosomal proteolysis and leucyl aminopeptidase activity, IRAP recruitment, as well as fusion of phagosomes with endosomes and lysosomes are all impaired. These data suggest that STIM1-dependent Ca²⁺ signalling promotes the delivery of endolysosomal enzymes to phagosomes to enable efficient cross-presentation.

STIM proteins sense Ca²⁺ depletion in the ER and activate store-operated Ca²⁺-entry (SOCE) in response, a process associated with dendritic cell functions. Here the authors show STIM1 is the major isoform controlling SOCE in mouse dendritic cells and provide a mechanism for its requirement in antigen cross-presentation.

UNC93B1 interacts with the calcium sensor STIM1 for efficient antigen cross-presentation in dendritic cells

Dendritic cells (DC) have the unique ability to present exogenous antigens via the major histocompatibility complex class I pathway to stimulate naive CD8⁺ T cells. In DCs with a non-functional mutation in Unc93b1 (3d mutation), endosomal acidification, phagosomal maturation, antigen degradation, antigen export to the cytosol and the function of the store-operated-Ca²⁺-entry regulator STIM1 are impaired. These defects result in compromised antigen cross-presentation and anti-tumor responses in 3d-mutated mice. Here, we show that UNC93B1 interacts with the calcium sensor STIM1 in the endoplasmic reticulum, a critical step for STIM1 oligomerization and activation. Expression of a constitutively active STIM1 mutant, which no longer binds UNC93B1, restores antigen degradation and cross-presentation in 3d-mutated DCs. Furthermore, ablation of STIM1 in mouse and human cells leads to a decrease in cross-presentation. Our data indicate that the UNC93B1 and STIM1 cooperation is important for calcium flux and antigen cross-presentation in DCs.

STIM proteins sense Ca²⁺ depletion in the ER and activate store-operated Ca²⁺ entry in response, a process associated with dendritic cell (DC) functions. Here, the authors show that optimal antigen cross-presentation in DCs requires the association of the chaperone molecule UNC93B1 with STIM1.

Purification of the Membrane Compartment for Endoplasmic Reticulum-associated Degradation of Exogenous Antigens in Cross-presentation

Dendritic cells (DCs) are highly capable of processing and presenting internalized exogenous antigens upon major histocompatibility class (MHC) I molecules also known as cross-presentation (CP). CP plays an important role not only in the stimulation of naïve CD8+ T cells and memory CD8+ T cells for infectious and tumor immunity but also in the inactivation of self-acting naïve T cells by T cell anergy or T cell deletion. Although the critical molecular mechanism of CP remains to be elucidated, accumulating evidence indicates that exogenous antigens are processed through endoplasmic reticulum-associated degradation (ERAD) after export from non-classical endocytic compartments. Until recently, characterizations of these endocytic compartments were limited because there were no specific molecular markers other than exogenous antigens. The method described here is a new vesicle isolation protocol, which allows for the purification of these endocytic compartments. Using this purified microsome, we reconstituted the ERAD-like transport, ubiquitination, and processing of the exogenous antigen in vitro, suggesting that the ubiquitin-proteasome system processed the exogenous antigen after export from this cellular compartment. This protocol can be further applied to other cell types to clarify the molecular mechanism of CP.

MHC Class I Cross-Presentation: Stage Lights on Sec22b

Efficient (cross-)presentation of antigens internalized by dendritic cells (DCs) requires vesicular communication between the early secretory and the endocytic/phagocytic pathways, in which the Sec22b protein has been suggested to have a key role. Here, we undertake a critical assessment of two new studies that evaluate the role of Sec22b using gene-targeted mice and come to contradictory conclusions.

The comings and goings of MHC class I molecules herald a new dawn in cross-presentation

MHC class I (MHC-I) molecules are the centerpieces of cross-presentation. They are loaded with peptides derived from exogenous sources and displayed on the plasma membrane to communicate with CD8 T cells, relaying a message of tolerance or attack. The study of cross-presentation has been focused on the relative contributions of the vacuolar versus cytosolic pathways of antigen processing and the location where MHC-I molecules are loaded. While vacuolar processing generates peptides loaded onto vacuolar MHC-I molecules, how and where exogenous peptides generated by the proteasome and transported by TAP meet MHC-I molecules for loading has been a matter of debate. The source and trafficking of MHC-I molecules in dendritic cells have largely been ignored under the expectation that these molecules came from the Endoplasmic reticulum (ER) or the plasma membrane. New studies reveal a concentrated pool of MHC-I molecules in the endocytic recycling compartment (ERC). These pools are rapidly mobilized to phagosomes carrying microbial antigens, and in a signal-dependent manner under the control of Toll-like receptors. The phagosome becomes a dynamic hub receiving traffic from multiple sources, the ER-Golgi intermediate compartment for delivering the peptide-loading machinery and the ERC for deploying MHC-I molecules that alert CD8 T cells of infection.

Intracellular recycling and cross-presentation by MHC class I molecules

Cross-presentation of internalized antigens by dendritic cells requires efficient delivery of Major Histocompatibility Complex (MHC) class I molecules to peptide-loading compartments. Strong evidence suggests that such loading can occur outside of the endoplasmic reticulum; however, the trafficking pathways and sources of class I molecules involved are poorly understood. Examination of non-professional, non-phagocytic cells has revealed a clathrin-independent, Arf6-dependent recycling pathway likely traveled by internalized optimally loaded (closed) class I molecules. Some closed and all open MHC class I molecules travel to late endosomes to be degraded but might also partly be re-loaded with peptides and recycled. Studies of viral interference revealed pathways in which class I molecules are directed to degradation in lysosomes upon ubiquitination at the surface, or upon AP-1 and HIV-nef-dependent misrouting from the Golgi network to lysosomes. While many observations made in non-professional cells remain to be re-examined in dendritic cells, available evidence suggests that both recycling and neo-synthesized class I molecules can be loaded with cross-presented peptides. Recycling molecules can be recruited to phagosomes triggered by innate signals such as TLR4 ligands, and may therefore specialize in loading with phagocytosed antigens. In contrast, AP-1-dependent accumulation at, or trafficking through, a Golgi compartment of newly synthesized molecules appears to be important for cross-presentation of soluble proteins and possibly of long peptides that are processed in the so-called vacuolar pathway. However, significant cell biological work will be required to confirm this or any other model and to integrate knowledge on MHC class I biochemistry and trafficking in models of CD8(+) T-cell priming by dendritic cells.

Dendritic cell maturation and cross-presentation: timing matters!

As a population, dendritic cells (DCs) appear to be the best cross‐presenters of internalized antigens on major histocompatibility complex class I molecules in the mouse. To do this, DCs have developed a number of unique and dedicated means to control their endocytic and phagocytic pathways: among them, the capacity to limit acidification of their phagosomes, to prevent proteolytic degradation, to delay fusion of phagosomes to lysosomes, to recruit ER proteins to phagosomes, and to export phagocytosed antigens to the cytosol. The regulation of phagocytic functions, and thereby of antigen processing and presentation by innate signaling, represents a critical level of integration of adaptive and innate immune responses. Understanding how innate signals control antigen cross‐presentation is critical to define effective vaccination strategies for CD8⁺ T‐cell responses.